1. Field of the Invention
This invention relates generally to power generating processes, and more particularly to a process for improving the thermal efficiency of a steam turbine power generating plant while simultaneously desalinating seawater or brine or purifying fresh water containing minerals, salts, and other dissolved solids. Efficiency is improved by utilizing a secondary economizer to increase high-pressure steam capacity and using low-pressure steam to replace high-pressure steam that is used for preheating and deareation of feedwater. Fresh water may be produced by multi-effect evaporation.
2. Brief Description of the Prior Art
As world population increases, demand for power and fresh water will also increase. Pollutants and drought result in a shortage of fresh water in many locations. Therefore, it would be desirable to provide a process utilizing desalination and distillation combined with power generation whereby demand for power and fresh water can be simultaneously satisfied.
Conventional methods for power generation include the steam cycle, cogeneration cycle, and the combined cycle. In the steam cycle, water is heated to produce steam at high temperature and pressure. The steam is typically superheated and expanded across a turbine to produce power. The steam will frequently be heated again and expanded across a turbine a second time. The steam will then be condensed at a low temperature and the cycle is repeated.
In a conventional cogeneration and combined cycle, high-pressure combustion products are expanded across a turbine to produce power. The combustion products are then heat exchanged against a boiler to produce steam. The resultant steam may then be sold or may be used to produce power.
Most previous methods of desalination have been stand-alone processes. Hence, they have focused upon energy efficiency to satisfy economics. Several of the commercial methods include reverse osmosis, evaporation, and vapor recompression. Dual purpose power plants have also been utilized.
Reverse osmosis is a technology wherein fresh water is extracted from saline water by pressure. This is accomplished by circulating saline water under high-pressure (i.e., 1000-2000 psig) around a loop. One portion of the loop is adjacent to a membrane. The membrane selectively allows water to pass through it while preventing the passage of most ions. Effectively, fresh water is squeezed from the saline water. Excellent energy efficiency can be achieved by this method. However, the membranes are prone to pluggage and in practice the fresh water produced is not completely free of dissolved salts. The present process, on the other hand, produces fresh water by a phase change and produces power.
Evaporation is the boiling of salinous water by the addition of heat followed by condensation of the resulting steam by heat exchange. Evaporators are typically classified as boiling or flashing. A large amount of heat input is required. Single stage evaporation is the least efficient of the existing methods. Multi-stage evaporation can have an energy cost similar to that of reverse osmosis. The present process uses evaporation wherein the waste heat used in the evaporation cycle is recovered.
Vapor recompression is a technology wherein water boils itself. This is accomplished by boiling water at low-pressure to produce water vapor. The water vapor is compressed and heated by doing work upon it. The heated water vapor is then condensed by heat exchange against the boiling water. The net result is that a phase change is accomplished by doing work. The latent heat of vaporization begins and ends at the same point within the vapor recompression process. In the present process, on the other hand, the latent heat originates and ends at different points outside the process.
Dual purpose desalination/power plants currently in use produce fresh water by using the exhaust steam as a source of heat for an evaporator. The exhaust steam is condensed against the boiler of the evaporator. Essentially, the power plant's condenser is replaced by the last effect of a multi-effect evaporator. This allows for the production of an enormous amount of fresh water. As the boiler duty increases with fresh water production, the temperature of the condensing exhaust steam also increases. This reduces the thermodynamic efficiency of the power plant providing the steam. In the present process efficiency of the power plant is not adversely affected by increasing the fresh water production rate. In fact, energy efficiency is improved by the recovery of additional waste heat from the stack exhaust.
Power generation using steam expansion is a common process. Condensate is deareated and preheated and then fed to a boiler and heated. Steam is removed from the boiler and typically superheated. It then expands across a turbine, thereby doing work. The steam is then condensed and recycled to the boiler. A moderate amount of liquid is intermittently withdrawn from the boiler to prevent sludge accumulation. Treated fresh water is added to the system to compensate for material losses. The present process, on the other hand, improves the thermal efficiency of a power plant by recovering additional waste heat from the exhaust gas using a secondary economizer.
There are several patents, which disclose various desalinating processes, some of which also generate power.
Simpson et al, U.S. Pat. No. 5,622,605 discloses a process which utilizes geothermal brine to generate power in a closed system with the exclusion of air to minimize corrosion. Steam from geothermal brine contains significant quantities of soluble salts including sodium and potassium chloride, calcium salts and iron and manganese salts, which have a strong corrosive action on turbine blades and related equipment. In this process, hot geothermal brine is flashed in a flash zone to form steam and concentrated brine and the steam is used to drive a power-generating turbine. The exhaust steam from the turbine is condensed and the major portion of the condensed steam is combined with the concentrated brine to form a restored brine, and the restored brine is returned to the geothermal hot brine well.
Kutchinson et al, U.S. Pat. No. 3,893,299 discloses a geothermal heat recovery process wherein hot water from a geothermal well is passed through successive flash chambers operating at successively lower temperatures and the steam from each flash chamber is passed in heat exchange relationship with a working fluid operating in a closed loop which is expanded in a power extracting gas expansion device for generating power. The hot fluid at the output of each heat exchange is either combined with the steam at the output of the next flash chamber or applied to the input of the next flash chamber with the hot fluid that is not converted to steam.
Spears, Jr., U.S. Pat. No. 4,078,976 discloses a potable recovery and power generating process which utilizes solar power for recovering potable water from salinous water. A portion of salinous water and an air stream are introduced into a solar radiation heat sink and heated water-containing air is withdrawn and condensed into potable drinking water. The heated salinous water is withdrawn from the solar radiation heat sink and recycled, and a part of the heated salinous water is flashed and the resultant vapor is passed through turbines to generate power and the exiting turbine vapors are cooled or condensed by contact with a second portion of the salinous water to recover addition potable water.
Pitcher, U.S. Pat. No. 4,267,022 and Gress, U.S. Pat. No. 4,310,382 disclose processes which utilize air as a working fluid for desalination and heat pumps for transferring latent heat associated with vaporizing or condensing water from one part of the process to another.
Mock, U.S. Pat. No. 4,276,124 and Elmore, U.S. Pat. No. 5,096,543 are essentially low-efficiency evaporator systems which utilize air as a working fluid to transport water vapor from one part of the system to another.
Becker, U.S. Pat. No. 3,557,863 discloses a process for obtaining fresh water from saline water by injecting saline water through nozzles into a hot high-pressure gas directed into an evaporation chamber to evaporate the saline water and generate a gas-vapor mixture and a precipitate. The gas-vapor mixture and the precipitate are separately withdrawn from the chamber. The gas-vapor mixture is engine expanded and then cooled to condense out fresh water.
Williamson, U.S. Pat. No. 3,489,652 teaches indirect contact of the saline water with the heat source in a heat exchanger at the first part of the process and then flash evaporating the saline water in successive stages in a multi-stage flash evaporator to produce a vapor fraction and a brine fraction in each stage and the brine is finally discharged as waste.
Martin, U.S. Pat. No. 3,950,949 discloses a process whereby steam is expanded to produce power. Low-grade heat such as solar or geothermal heat is used to heat a liquid to saturation temperature in a first boiler or heat exchanger and the vapor leaving the boiler or heat exchanger passes through a vapor phase regenerator which provides a first increment of superheat and to a superheater where it is superheated by a high-grade heat source (focused solar radiation or combustion). The superheated steam or vapor leaving the superheater is expanded in a turbine (expansion engine). The superheated steam expanded in the turbine is passed through the vapor phase regenerator where it heats the fluid which is being fed to the superheater and then passes to a liquid phase regenerator which serves to preheat the liquid being fed to the boiler or heat exchanger.
Kestner, British Patent 165,066 discloses a process using a group of boilers fed with distilled water combined with a distilling boiler which is not fed with distilled water, or an evaporator (low-pressure boiler) heated by steam from the group of boilers. Distilled water obtained from natural water and purified by the blow-down from the boilers is used as make-up for the liquid loss due to the blow-down.
The present invention is distinguished over the prior art in general, and these patents in particular by a process and apparatus for improving the thermal efficiency of a steam turbine power generating plant while simultaneously desalinating seawater or brine and purifying water which contains minerals, salts, and other dissolved solids. Exhaust gases from a power plant is heat exchanged against water in a secondary ecomomizer which circulates water at a temperature near, or slightly above the dewpoint of the combustion exhaust of the high-pressure boiler. The heated water is flashed to produce low-pressure steam. The low-pressure steam is condensed against the last effect of a multi-effect desalinization unit. Steam from the first effect of the desalination unit is condensed against steam condensate from the power plant turbine to preheat the condensate and thereby recover heat from the power plant's exhaust gas. Salinous water is fed to the multi-effect desalinization unit to produce fresh water and a concentrated brine. The low-pressure steam is used to replace high-pressure steam that is used for preheating and deareation of feedwater for the boiler.